1. Field of the Invention
The invention relates to metal film resistors and in particular to resistors having two or more layers of a metallic film deposited on an insulative substrate, wherein at least two different metallic compositions are deposited alternately in the sequence of layers. Alternating metallic compositions in a layered resistive film structure provides a technique for controlling the TCR and the TCR Slope of the resistive film.
2. Description of the Prior Art
Metal film resistors are typically made by single target sputtering of a metallic alloy composition on an insulative substrate and subjecting the resulting sputtered substrate to a heat treatment in air at approximately 300.degree. C. Typically either a ceramic core or a ceramic chip is utilized as the substrate. The resistive films used are typically alloys of nickel and chrome with some other metals used in lesser percentages. Sputtered or evaporated NiCr alloys are widely used as deposited resistive film.
The desired TCR is obtained by heat treating the resistive film. The range of time and temperature for the heat treatment is usually a function of the desired temperature coefficient of resistance (TCR) of the resistor. During the heat treatment there is a growth of crystals in the bulk of the resistive film applied to the substrate; the larger the crystals, the more positive the TCR will be. However, during heat treatment crystals on the surface of the metal film break down and surface oxidation takes place, causing the TCR to be less positive in that area. With the addition of a heat treatment to the process of making resistors, the net effect is that for most resistors the TCR will be positive because crystal growth is promoted in the bulk of the metal film. To prevent the TCR from becoming too positive, contaminants can be introduced into the sputtering process. Reactive sputtering can be used concurrently for TCR control. However, only TCR is controlled thereby, not TCR Slope.
One problem with prior art metal film systems for resistor applications is that the TCR Slope cannot be controlled. Controlling the TCR Slope enables one to produce a resistor whose operation is more independent of temperature and is therefore more stable. Ideally, a TCR of 0 (zero) and a TCR Slope of 0 (zero) is desirable. To control to the TCR Slope and thereby obtain a TCR approaching 0 (zero) over a wide range of factors, a layering of metallic films of differing material composition has been found to be effective. The present invention is directed to a layered metal film resistor having significantly higher stability than prior art metal film resistors and having a significantly higher resistance in ohms per square than prior art metal film resistors.